In the Standard model the polymer chain slides out of the pull rail as the screen is closed.
If the width of the screen is wider than the height, there will not be enough room for all the chain in the pull rail. In this case a pull rail is provided on both sides of the screen, so the chain will slide in & out of both pull rails. We refer to this as the Extra Large model. According to the chart below, your screen will be the Standard (single pull rail) or Extra Large (double pull rail) configuration.